One known type of powered medical instrument is a dental drill, including a handpiece containing an electric motor, a separate motor control unit detachably coupled to the handpiece, and a progressively actuatable foot switch used by an operator to vary the motor speed.
Conventional instruments of this type use brushless motors contain Hall sensors which are used to monitor motor operation. However, the handpiece containing the motor must be periodically subjected to high temperatures for purposes of sterilization, for example by being placed in an autoclave. This presents a problem, in that the high temperatures of an autoclave tend to destroy the Hall sensors in the motor. One known approach for protecting the Hall sensors is to hermetically seal them, but the sealed sensors are relatively large and prevent the motor from being relatively compact and lightweight, which is desirable in a handpiece.
Brushless motors which do not have sensors have been developed for other applications, such as rotationally driving the hard disk drive of a personal computer. However, these other applications typically involve a relatively simple motor control situation, because the motor is always operated at a predetermined fixed speed. In contrast, a powered medical instrument such as a dental drill must be capable of operation through a range of motor speeds and loads.
A further consideration is that, as digital technology has improved, the doctor or dentist using a dental drill is typically permitted to manually select a maximum motor speed for a given drilling operation, and during the drilling operation is able to watch the actual motor speed on a digital display. However, manufacturing tolerances of the motor and various components in the motor control arrangement can cause the actual speed to vary somewhat from the specified speed. For example, the motor speed constant, which is a function of manufacturing tolerances, may vary by 10% from motor to motor. While the actual speed may be reasonably close to the specified speed, the precise accuracy inherent in a digital display tends to make even small deviances appear significant, suggesting to the operator that the system is not fulfilling its responsibility of operating the motor exactly at the specified speed. Although it is theoretically possible to minimize such deviances by holding all critical components to very tight manufacturing tolerances, this significantly increases the cost of these components, and thus the cost of the overall system.
Still another consideration is that the electric motor used in a dental drill or similar medical instrument is often capable of producing torques which would break certain components within the drive train of the handpiece, and it is thus important to be able to limit motor torque to a value which avoids breakage. According to the present state of the art, the electric motor is usually operated by a motor control invertor having several pairs of transistors arranged in a totem pole configuration and controlled by complementary pulse width modulated control signals. Torque limiting schemes have previously been developed, but often limit the torque to a predetermined value which cannot be varied, and often have the effect of causing the transistors of the invertor to run in a linear mode rather than a switching mode, causing the transistors to generate more heat and thus necessitating the use of heat sinks and/or larger packages.
In view of the foregoing, one object of the present invention is to provide a powered medical instrument which utilizes a brushless sensorless motor and provides variable speed operation of the motor.
A further object is to provide a powered medical instrument having an arrangement for conforming actual motor speed to a digitally specified speed without requiring the use of strict manufacturing tolerances for the motor and certain components of the motor control arrangement.
A further object is to provide a powered medical instrument having a torque limiting arrangement which permits torque to be limited to a range of values while ensuring that the drive elements of an invertor controlling the motor always run in a switching mode and never in a linear mode, thereby substantially eliminating heat dissipation and avoiding heat sinks, while allowing tighter packaging.